A pacemaker is an implantable medical device that recognizes various arrhythmias such as an abnormally slow heart rate (bradycardia) or an abnormally fast heart rate (tachycardia) and delivers electrical pacing pulses to the heart in an effort to remedy the arrhythmias. An ICD is an implantable device that additionally recognizes atrial fibrillation (AF) or ventricular fibrillation (VF) and delivers electrical shocks to terminate fibrillation. Pacemakers and ICDs detect arrhythmias by sensing internal electrical cardiac signals using leads implanted within the heart. The internal signals comprise an intracardiac electrogram (IEGM). Within the IEGM, the normal contraction of atrial heart muscle tissue appears as a P-wave whereas the normal contraction of ventricular muscle tissue appears as an R-wave (sometimes referred to as the “QRS complex”). More specifically, the P-wave corresponds to the electrical depolarization of atrial tissue and the R-wave corresponds to the depolarization of ventricular tissue. The subsequent electrical repolarization of the ventricular tissue appears within the IEGM as a T-wave. Strictly speaking, P-waves, R-waves and T-waves are features of a surface electrocardiogram (EKG or ECG). For convenience, the terms P-wave, R-wave and T-wave are also used herein to refer to the corresponding internal signal component.
Pacemakers and ICDs are often configured to be used in conjunction with a programmer that allows a physician to program the operation of the implanted device to, for example, control the specific parameters by which the device detects arrhythmia and responds thereto. For example, the programmer may allow the physician to specify the sensitivity with which the implanted device senses electrical signals within the heart and to further specify the amount of electrical energy to be employed for pacing the heart. Additionally, the programmer may be configured to receive and display a wide variety of diagnostic information detected by the implanted device, such as graphs of the IEGM. In addition, the programmer may operate to analyze data received from the device to assist the physician in rendering diagnoses as to possible arrhythmias and to then assist the physician in programming the device to provide appropriate therapy.
Current state of the art implantable cardiac stimulation devices may have dozens or hundreds of programmable parameters that can be individually programmed using the external programmer. The programmable parameters permit the operation of the cardiac stimulation device to be tailored to the needs of the particular patient to provide optimal therapy while minimizing the risk of any unnecessary therapy. Unfortunately, it is often difficult to predict the effect within a given patient to a selected set of parameter settings. Hence, a potentially viable set of parameters is chosen by the physician, the implantable cardiac stimulation device is programmed using the selected set of parameters and then the patient is sent home. Weeks or months later the patient returns to the physician's office for a follow-up appointment so that they physician may evaluate the effect of the selected parameters. Typically, the follow-up evaluation consists of the physician making judgments based upon a review of diagnostic information provided by the implanted device (including the IEGM) in combination with a surface EKG provided by a separate surface EKG unit. As part of the review, the physician may also compare new surface EKGs with recorded surface EKGs from previous sessions. In any case, the physician adjusts the programming parameters of the implanted device to improve therapy delivered to the patient. Again, the patient is sent home for several more weeks or months until another follow-up visit. This cycle may be repeated numerous times before optimal device settings are determined by the physician.
To obtain a surface EKG, typically, ten electrodes are manually attached the skin of the patient in the configuration shown in FIG. 1. The surface EKG derived from the ten electrodes is referred to as a “12-lead” EKG because twelve signals are derived from the ten electrodes—including signals from each of the individual electrodes plus signals between certain pairs of electrodes. More specifically, the ten electrodes include four limb electrodes and six “chest” electrodes. The chest electrodes are labeled: V1-V6. The limb electrodes are: RA (right arm), LA (left arm), LL (left leg) and right leg (RL), the last of which is optional. The chest electrodes provide one signal per electrode, referred to as the V1-V6 signals. The RA, LA and LL limb electrodes also provide one signal per electrode, referred to as the aVR, aVL and aVF signals (with F signifying foot as opposed to leg.) Finally, the difference between each pairing of the RA, LA and LL limb electrodes is considered a separate “lead” (referred to as the Einthoven leads I, II and III) and hence provide the last three signals of the surface EKG. The twelve signals of the surface EKG are summarized in TABLE I, along with the electrodes from which the signals are derived.
TABLE ISURFACE EKGPHYSICAL LEADSSIGNALSV1V1V2V2V3V3V4V4V5V5V6V6LA - RAILL - RAIILL - LAIIIPAaVRLAaVLLLAvf
The twelve signals are combined to yield a single surface EKG, an example of which is shown in FIG. 2. It is particularly important for the physician to review the surface EKG during follow-up programming sessions. See: “The Paced Electrocardiogram Should No Longer Be Neglected in Pacemaker Follow-Up”, by S. Serge Barold; Paul A. Levine; I. Eli Ovsyshcher, PACE 2001; 24: 1455-1458. However, the need to manually attach and remove each of the surface EKG electrodes from the patient during each follow-up session is a burden to the physician (or his or her staff) and a considerable inconvenience to the patient. In many cases, the skin of the patient must be shaved and sanded in the locations where the electrodes are to be attached to provide adequate electrical conduction. This can be quite uncomfortable and, in some cases, embarrassing for the patient. Moreover, the time required to attach and then remove the electrodes adds to the overall cost of the follow-up session. Also, from one follow-up session and another, the electrodes may not be placed at the exact same locations on the patient, thus resulting in somewhat different surface EKGs and making it more difficult for the physician to properly identify actual differences in cardiac signals of the patient from one session to the next.
As can be appreciated, it would be desirable to eliminate the need for attaching the electrodes of the surface EKG to patients during follow-up sessions to thereby reduce the cost and inconvenience to the patient and to eliminate problems resulting from differing electrode placement. One proposed solution is to emulate the surface EKG using internal electrical cardiac signals sensed by the implanted device so that, during a follow-up session, a separate surface EKG system is not required and external electrodes need not be attached to the patient. One effective technique for emulating a surface EKG using internal electrical signals is described in U.S. patent application Ser. No. 10/334,741 to Kroll et al., entitled “System and Method for Emulating a Surface EKG Using Implantable Cardiac Stimulation Device”, filed Dec. 30, 2002, which is assigned to the assignee of the present application and is incorporated by reference herein. With the technique of Kroll et al., each of the separate signals of the 12-lead EKG are individually emulated based on IEGM signals derived from implanted electrodes.
Although Kroll et al. provides a powerful technique for emulating all twelve signals of the EKG, in many cases, it is not always necessary for the physician to separately review the individual signals and only a combined surface EKG is needed. Hence, rather than separately emulating individual 12-lead EKG signals and then combining the signals into single surface EKG, it would instead be desirable to provide simpler and more direct techniques for emulating a combined surface EKG.
It should be noted that there are some existing techniques that serve to directly emulate a single combined surface EKG. See, for example, U.S. Pat. No. 5,740,811 to Hedberg et al., entitled “Device and Method for Generating a Synthesized ECG”. With the technique of Hedberg et al., a neural network is employed to convert electrical signals derived from implanted electrodes into a single emulated or “synthesized” surface EKG. Although the technique of Hedberg et al. directly emulates a single combined surface EKG, it appears to be computationally intensive. Accordingly, it would be desirable to provide improved techniques for emulating a single combined surface EKG that are not computationally intensive.